The Neighborhood Bully Predaceous Diving Beetle Ruthless Hunter In Water World

December 14, 1985|By Denise Salvaggio of The Sentinel Staff

Everyone is familiar with the saying ''When the going gets tough, the tough get going.'' Still, this pithy thought fails to tell the whole story. As is often the case, the going gets tough because even tougher individuals have created the situation in the first place. For example, if you think ponds, lakes or streams are peaceful, quiet places, take a good look at the predaceous diving beetle (family Dytiscidae) and reconsider.

Also known as the true water beetle, this 1.5-to-35-millimeter insect is the undisputed neighborhood bully in its fluid environs. And the predaceous diving beetle is in no danger of losing its swagger, since there are more than 4,000 species spread throughout the world to wreak havoc.

The flat-bodied, oval-shaped beetle's black, brown or yellowish color is outlined in lighter-hued markings -- certainly not a description that would strike fear into other insects and small vertebrates such as fish. But as its name emphatically states, neither the adults or larvae (aptly called ''water tigers'') are vegetarians.

Nor do the members of either generation sit placidly by and wait for prey to drift their way -- as some amateur naturalists have discovered the hard way. Naive collectors who place predaceous diving beetles in an aquarium among other marine life discover that the voracious newcomers seem related to the piranha, having destroyed or mutilated the stock within hours. Even within less confined quarters, they do not hesitate to attack an animal larger than themselves.

The predaceous diving beetle is well designed to be a ruthless hunter. The larva is long and slender, with sickle-like jaws. The powerful jaws have canals through which the creature pumps digestive juices into its victim and sucks out the digested tissues. When the young water tiger has a leisurely swim, it uses its legs for propulsion, but it can make a sudden spring by throwing itself into serpentine curves. The larva pupates beneath moist ground near the water's edge.

The adult beetle owes its diving talent to its long pair of hind legs, which are flattened and fringed to provide surface area that aids in flotation. The spiracles (openings through which the beetle breathes) are on the abdomen just under the tips of the elytra (wing covers).

When in a resting position -- below the water surface or on an incline with the head down -- the insect raises the tips of the elytra and breathes. When ready to dive, it stores a supply of air under the wings for breathing while underwater.

The predaceous diving beetle is so hardy that one individual was kept in captivity for three and a half years. Its longevity in the wild may not be much shorter because of its fierce nature and defensive fluid discharged from behind the head and anal glands that deter enemies.

Because the predaceous diving beetle eats fish and competes with them for food and space without consenting to become food for fish themselves on occasion, it is not considered beneficial. When this insect makes the going tough, everybody else gets out of the way.